1. Field of the Invention
This invention relates generally to the deaeration of hydraulic fluid, and more particularly to a vacuum deaerator which provides for evolving air out of solution so as to improve hydraulic system efficiency and effectiveness.
2. Description of the Prior Art
Hydraulic fluids normally contain dissolved air. This does no harm provided the air remains in solution. Under certain circumstances undesolved or free air may be found, and this could be detrimental to the performance of the system and to the life of certain system components, and could cause excessive noise. The interaction between the properties of the hydraulic fluid and the characteristics of the circuit will determine whether aeration problems arise.
The amount of dissolved air is determined by Henry's law, which states that the solubility of a gas in a liquid is directly proportional to the absolute pressure. At atmospheric pressure and room temperature, most mineral based hydraulic fluids contain about 8 to 10 percent by volume of dissolved air. With this air content, if the oil is then subjected to an increased pressure, it will become relatively unsaturated and will tend to dissolve any free air that is available. If the same oil with the same initial air content is exposed to a reduced pressure, it will become relatively supersaturated and air will tend to come out of solution. In the absence of turbulent mixing, this process will be relatively slow for small pressure differences. For sudden and large reductions in pressure, dissolved air may come out of solution more rapidly, indeed with almost explosive violence.
Aeration, or bubble formation, in oil may occur in various ways; as the result of air coming out of solution in regions of low pressure, by entrainment in oil splashing or cascading to a sump, or from leaks in pipes or system components. Air also may be trapped in the system during the filling process unless bleeding is carried out, and the pockets may be dispersed throughout the oil when flow commences. Once free air bubbles are formed, they will continue to grow if the pressure is lower than saturation pressure, or will gradually dissolve if it is higher. The behavior of the hydraulic system under these conditions is relatively inefficient.
Many hydraulic systems are so designed and operate in such a manner that air is alternately dissolved in the oil at relatively high pressure and evolved from the oil at relatively low pressure. This type of operation can give rise to malfunctions of the system components, especially in the case of fluid control systems.
In recent years there has been interest in improving the efficiency of hydraulic systems by deaerating oils used therein. One way in which deaeration has been attempted involves improved sump design; another the use of a wire mesh screen in a sump for separating free air from oil flowing through the sump. Other improvements involve centrifugal or vortex devices.
Prior art systems generally have either been complicated or have been relatively inefficient, and thus ineffective for significantly reducing the amount of air in solution. There remains a continuing need and desire in the art for improved deaeration of oil, either for immediate use in a hydraulic system or for storage in a sealed container for later use.
Removal of dissolved air from oil is most effectively accomplished by two means: decreasing temperature and reducing pressure. Of these, reduction of pressure is more effective whenever extensive removal of air is desired.
The improved vacuum deaerator herein uses a vacuum pump to lower the pressure on oil and also to propel the oil. The deaerator includes a spiral passageway which oil enters at the periphery and from which discharge occurs at the center. The passageway is provided with a flow restricting device at the ingress thereof. This device induces an initial pressure drop. There are two discharge ports at the egress; an upper discharge port for air and a lower discharge port for oil. A container may be filled with oil from the oil discharge port, or alternatively, deaerated oil may be used directly in an associated hydraulic system. The container and the spiral passageway also are subjected to reduced pressure, induced by a single vacuum pump. The deaerator has no moving parts other than this vacuum pump.